1. Field of the Invention
This invention relates to low-density polyethylene and more specifically, it relates to the manufacture of low-density polyethylene in stirred autoclaves.
2. DESCRIPTION OF THE PRIOR ART
The industrial high pressure processes for the manufacture of low-density polyethylene differ substantially only in the design of the reactors. Depending on the principle of their construction, the reactors are commonly classified as tubular reactors and (stirred) autoclaves.
Tubular reactors are characterized by a very high L/D ratio (L=length, D=diameter), values between 10,000 and 50,000 being the rule. As a result of this geometry, back-mixing of the material being reacted in such reactors is extremely slight, that is to say, a very close approximation to plug flow is achieved.
The stirred autoclaves in high-pressure polyethylene processes in general have a L/D ratio of &lt;20. Back-mixing in such autoclaves is significantly affected by the L/D ratio. At L/D ratios of &lt;3-4 and with a corresponding design of the stirrer, practically complete back-mixing is obtained. On the other hand, in the case of higher L/D ratios and when suitable inserts are used, back-mixing can be restricted in a controlled manner.
The degree of back-mixing is very important from two points of view: (a) process stability and (b) product properties.
Process stability is extremely important in high-pressure polyethylene processes because of the extreme reaction conditions during the manufacture of low-density polyethylene (pressure range of normally 1,500-3,000 bars and temperature range of 150.degree.-300.degree. C.), the high heat of polymerization of ethylene (approx. 800 kcal/kg or 3.3. MJ/kg) and the tendency of ethylene to decompose explosively at high pressures and temperatures between 300.degree. and 400.degree. C. (see Chemical Engineering Science, 1973, Vol. 28, 1,505-1,514).
Good control of such extreme reaction conditions thus makes a substantial contribution to process stability. For example, a high degree of process stability also means that the number of emergency blow-down to atmosphere and the associated dangers and consequences, such as ignition of the ethylene/air mixture and environmental pollution by noise and harmful substances, are substantially reduced. In addition to effective control instruments, such as sophisticated control loops for the temperature and pressure, it is primarily intense mixing in the reaction zone which assists control of the reaction, because it effects a rapid and uniform distribution of all the reactants. In this way, possible inhomogeneities in the reaction zone, which can result from incomplete mixing caused by fluctuations in metering and/or flow conditions, are excluded a priori and the formation of so-called "hot spots", which can be the starting point for the explosive decomposition of ethylene, is avoided.
According to prevailing opinion a reaction system exhibits practically complete back-mixing when the mixing time, i.e. the average duration of a mixing cycle, is equal to or less than a tenth of the average residence time of the reaction mixture.
The patent of Christl et al., U.S. Pat. No. 2,897,183, describes a constant environment reaction system in terms of end-to-end mixing of between 30 and 200 average number of cycles of the reacting mixture in accord with the equation: NC=G.sub.r /G.sub.f, in which NC is the number of cycles, Gr is the end-to-end circulation in pounds per hour, and Gf is the monomer(s) feed rate in pounds per hour. In the case of autoclaves, the degree of end-to-end mixing ("back-mixing") depends essentially on geometry of the vessel and the internal arrangement of the vessel (agitator, baffles, etc.). In tubular reactors, back-mixing is virtually prevented as a result of the design. However, complete back-mixing, which is desirable from the point of view of process stability and flexibility, entails some limitations with respect to certain product properties.
It is known that the end-use properties of low-density polyethylene are substantially influenced not only by the melt index and the density, which normally are set by the reaction parameters, that is to say the pressure, the temperature and the concentration of molecular weight regulator, but also by molecular factors, such as long-chain branching .lambda. and non-uniformity ##EQU1##
(Mw=weight average and Mn=number average molecular weight), and, in addition to the reaction parameters mentioned, reaction conditions attributable to the homogeneity or inhomogeneity of the reaction environment are responsible for these molecular factors (compare Die Angewandte Makromolekulare Chemie 40/41 (1974) 361-389 (No. 586)). Such reaction environments are characterized by the presence or absence of temperature gradients, concentration gradients and pressure gradients during the course of the reaction. Temperature gradients and concentration gradients are mutually interdependent and are the consequence of incomplete back-mixing. The pressure gradient is normally given by the lay-out of the reaction system and is restricted to tubular reactors. In the case of complete back-mixing (constant environment) none of the three gradients is possible.
A relationship between the temperature gradient, concentration gradient, and pressure gradient on the one hand and, on the other hand, the properties of the product arises from the fact that all the reaction steps involved in the polymerization, such as initiation reactions, growth reactions chain-transfer reactions and chain-stopping reactions, depend on the temperature, concentration and pressure in various ways. For example, a high concentration of polymer favors the long-chain branching .lambda. which, in order to take place, requires the interaction of a free radical with a polymer chain. Under comparable reaction conditions (pressure, temperature and identical final conversion), more long-chain branches are consequently produced in a homogeneous reaction system with a constant polymer concentration (final concentration) than in an inhomogeneous reaction system in which the polymer concentration rises from a low initial value to the final value. The rheology, crystallizability, melt relaxation and other properties of the two products differ in spite of identical melt index.
For certain fields of application, for example, the melt-coating of paper, board, metal foils and the like, products having high values of .lambda. and U are advantageous because their relaxation behavior leads to low "neck-in" values (narrowing of the melt curtain after leaving the extrusion equipment). In other fields of application, for example the blowing of thin film, products having low .lambda. and U values are advantageous since they possess good rheological properties, drawability and optical properties.
This comparison demonstrates the influence of back-mixing in the reaction system on the product properties. In tubular reactors back-mixing is virtually prevented as a result of the design. In the case of autoclaves, the degree of back-mixing essentially depends on their geometry and their internal arrangement.
Although a practically completely back-mixed reaction system should be given preference from the point of view of process stability as far as certain product properties are concerned the arguments for a non-backmixed reaction system prevail.
This knowledge has led to a number of proposals (U.S. Pat. Nos. 3,178,404; 3,536,693; 3,575,950; 3,692,763 and 3,875,128, British Patent Specification No. 1,071,305 and Belgian Patent Specification No. 710,392) with the aim of improving certain properties of the product by carrying out the polymerization in two or more reaction zones operating at differing (rising) temperatures. This is achieved largely by modifying the reactor to provide separate reaction zones, with no back-mixing in the reactor betweeen zones, for example by means of inserts and special types of stirrer. In order to obtain the temperature zones, U.S. Pat. No. 3,178,404 proposes a two-zone reactor with substantially no back-mixing between the zones as the preferred solution and, moreover, indicates the possibility of using an elongated tubular reactor or separate reactors. It is advantageous that the initiators employed for the different reaction zones have differing activities and are adapted to the particular temperatures. U.S. Pat. No. 3,178,404 claims the use of caprylyl peroxide for the reaction zone having the lower temperature (130.degree.-190.degree. C.). In the British Patent Specification No. 1,071,305 the combination of two slender autoclaves (L/D 11:1 to 20:1) is described which are operated under very different reaction conditions in such a way that the high-molecular weight reaction product of the first autoclave is led into the lower zone (mixing zone) of the second autoclave. ("wax reactor"). In U.S. Pat. No. 3,875,128 a combination of two or more slender autoclaves (L/D 5-20) is claimed with cooling units positioned between the autoclaves, in order to increase the conversion of the polymerization. The resulting considerable losses of pressure and possible formation of deposits in the coolers are taken into account.
The advantages of these proposals are, however, offset by two substantial disadvantages which are caused by the loss of practically complete back-mixing: 1. a reduction of process stability; 2. the loss of the possibility of producing products the optimum properties of which are achieved when the reactor is practically completely back-mixed.